(1) Field of the Invention
The present invention relates to a program execution control device for a computer system, and more particularly, to a program execution control device for copying an object program stored in external storage means to a main memory and controlling execution of the program.
(2) Description of the Related Art
Binary-coded object programs executable by a computer system are created in the following manner. First, programs called source programs described in a source language (programming language) are compiled using a compiler, to prepare object modules. Then, using a linker, the object modules are combined with predetermined libraries to produce object programs. During the combining operation, the linker calculates the checksum of the entire combination of the object module and library and affixes the calculated checksum to a predetermined location in the object program as auxiliary data.
Object programs created in this manner are generally constructed in a dynamically relocatable fashion. Thus, when an object program stored in an external storage such as a hard disk is executed, an operating system (OS) loads the object program into the main memory while relocating the object program, or more specificaly, while converting address information contained in the object program. The OS then sets registers in the processor that are necessary for the execution of the object program.
In recent years, programs called computer viruses have spread, causing damages such as destruction of important files. If a computer virus infects a certain object program, the program runs abnormally, presenting a phenomenon that a strange screen appears at the display device upon lapse of a certain period of time or part or all of the files stored in the external storage or the like is destroyed. Computer viruses can infect not only the object programs as aforesaid but other object programs such as OS or shell, causing even greater damage. In the event a computer virus enters a computer system serving as a host station of a networked on-line system, such as the one used in banking facilities, the damage caused is unrecoverable.
The phenomenon described above can also occur in situations where a bit loss has occurred in the object program stored in the external storage, due to a change with time of the external storage or the like, or where a burst error occurs when the object program is copied from the external storage to the main memory, due to external noise such as alpha rays or impulses.
With conventional computer systems and OS, it is not possible to determine in advance whether the object program copied from the external storage to the main memory is damaged by a computer virus, or whether a bit loss or burst error has occurred in the object program due to time-based change, external noise or other causes.